1. Field of the Invention (Technical Field)
The present invention relates to the field of short pulse ring lasers and applications therefor.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
The gyroscope (gyro) has been in use for navigational purposes since the 1920""s, its accuracy improving as technology advanced. Today""s state-of-the-art gyroscopes use two counter-propagating beams of continuous wave (CW) light in a ring configuration and take the beat frequency xcex94"ugr" between the two beams to be a measure of the rotation rate xcexa9.
The principle behind the laser gyro is known as the Sagnac effect. This effect is the phase shift induced by a light beam as it completes a loop, when the plane of the loop is given a rotation. Consider the case where the loop is part of a laser cavity. One condition for lasing is that the cavity length which, to be precise, should be the optical path length of the laser cavity, and the cavity length must be an integral multiple of the wavelength of the light. Hence, the lasing frequency will adjust itself to match the cavity length. When the ring experiences a rotation (xcexa9) or other non-reciprocal effects, the light travelling in one direction senses a different optical path length than the one travelling in the opposite direction. The light which travels against the rotation sees a slightly shorter path, and hence its frequency is upshifted while the reverse is true for the light travelling with the rotation. By recombining these two beams outside of the ring cavity, one observes a beat frequency xcex94"ugr", which is the difference of the two light frequencies. The following relation is easily derived from consideration of path length changes:       υ    0    =                              4          ⁢          A                          P          ⁢                      xe2x80x83                    ⁢          λ                    ⁢              xe2x80x83            ⁢      Ω        =          R      ⁢              xe2x80x83            ⁢      Ω      
where A is the area of the ring which is perpendicular to the rotation axis, xcex is the wavelength of the light within the ring and P is the ring perimeter. From this equation, note that a) the beat frequency due to rotation (Sagnac effect) is directly proportional to xcexa9 with the proportionality constant R being determined by the cavity geometry and that b) the plot of xcexa9 versus xcex94"ugr" should pass through the point (0,0). Deviations from these two conclusions are addressed by the present invention.
The first problem is that of linearity. It was found from the beginning that the counter-propagating beams of light become locked in frequency when the rotation rate was small, leading to the existence of a xe2x80x9cdead bandxe2x80x9d where the gyro has zero response. The solution currently common in the art is to dither the ring thereby shaking the two beams out of the lock-in regime at small rotation rates. Of course, one consequence is that the gyro becomes more cumbersome due to the mechanical dithering devices. In addition, the gyro response near the dithering frequency does not reflect the xe2x80x9cactualxe2x80x9d rotation rate xcexa9. Hence, the linear response is still not assured.
If the scattering that causes the lock-in is phase conjugated, the dead band can be reduced. U.S. Pat. No. 4,525,843 to Diels, entitled xe2x80x9cRing Laser with Wavefront Conjugating Beamsxe2x80x9d, discloses a phase conjugating coupling element inside the laser cavity to reduce lock-in. This patent has not been implemented in standard CW laser gyros, but provides background related to laser gyros based on short pulses.
Since the lock-in results from the scattering from either circulating beam into the one circulating in the opposite direction, one solution to eliminate the coupling is to use short pulses, and to ensure that they do not encounter a scattering medium where they cross. One implementation presently put in practice with dye lasers and Ti:sapphire lasers is to use phase conjugated coupling, such as disclosed in the ""843 patent, at one crossing point, and ensure that the other crossing point is in air as disclosed by U.S. Pat. No. 5,363,192 to Diels et al., entitled xe2x80x9cMode-Locked Active Gyro Solid State Lasersxe2x80x9d.
Some possible implementations of the ""192 patent use saturable absorbers at a crossing point. The scattering associated with these media can cause lock-in, which can be eliminated by moving the saturable absorber transversally to the laser beam as disclosed in U.S. Pat. No. 5,367,528 also to Diels et al., entitled xe2x80x9cMotion Induced Elimination of Dead Band in a Short Pulse Laser Gyroxe2x80x9d. The ring laser that is mode-locked with a nonlinear substance of the present invention improves upon these prior art patents and provides applications for the unique bi-directional short pulse ring laser of the present invention.
In the case of a short pulse laser gyro as in the ""192 patent, an artificial rotation can be obtained by inserting an electro-optic modulator (phase modulator) in the cavity, and pulsing it at the cavity round-trip time, in such a manner as to give a different cavity (optical) length for the pulses circulating clockwise and counterclockwise as disclosed in U.S. Pat. No. 5,251,230 to Lai et al., entitled xe2x80x9cResonant Cavity Dither with Index of Refraction Modulator.xe2x80x9d As a result of this difference in cavity length, the two trains of pulses will exhibit a different frequency, resulting in a beat note undistinguishable from that corresponding to a rotation. A sufficient artificial rotation will force the laser gyro out of its xe2x80x9cdead bandxe2x80x9d. This type of electro-optic dither can be implemented with a short pulse laser gyro of the type mentioned in the ""192 patent.
Commercial laser gyros are all Hexe2x80x94Ne gas lasers, operating in continuous mode. These gyros are plagued by a dead band, even using very expensive optics that have much less scattering than normal mirrors. Because of this dead band, one has to physically move the laser in order to have the laser operate outside of that range. It defeats the purpose of making a laser gyro, if moving parts are to be used. Further, vacuum tubes such as Hexe2x80x94Ne lasers are an obsolete technology. The electrodes have a short lifetime and the laser itself has very low efficiency.
The embodiments of the present invention make it possible to use much more efficient solid state lasers, in particular diode pumped lasers. The dead band is suppressed without introducing a motion of the whole laser. In one of the embodiments, a small motion of one very small component (a solid state saturable absorber, or nonlinear crystal, or a flowing absorber, of a flowing nonlinear liquid) of the laser is used to eliminate a possible dead band. In another embodiment (electronic dithering), the dead band is completely eliminated electronically. In yet another embodiment, the laser is actively mode-locked by one or two modulators. Several applications are provided as well including detection of magnetic susceptibility; measurement of small displacements, measurement of the time derivative of n(t); measurement of high voltage; measurement of the derivative of voltage; and measurement of magnetic and electric fields.
The present invention is a method of making a bi-directional pulse ring laser and comprises the steps of providing a substance with an index of refraction that is light intensity-dependent; locating the substance in proximity to a beam waist of the laser cavity; and interacting bi-directional light pulses such that they are phase conjugated. The method further comprises the steps of altering the beam diameter within the cavity with the self-lensing effect of the substance and producing bi-directional short light pulses by inserting an aperture in the cavity where the beam diameter decreases with intensity. The laser source comprises either a Ti:sapphire laser, an Nd:vanadate laser, or a Cr:LISAF laser. The nonlinear substance that has an index of refraction that is light intensity-dependent can be either a nonlinear crystal or fluid. The fluid can be provided in a glass cell or free-flowing jet.
The method further comprises reducing residual dead band beyond observable limits, in particular less than that corresponding to the rotation of the earth, in the bi-directional pulse ring laser. Reducing the dead band can comprise electronically modulating with non-constant modulation. By modulating with non-constant modulation, a beat frequency spectrum is provided that is due to rotation alone. The non-constant modulation can comprise electronically modulating with a signal that is symmetrically switched between opposite polarities; this signal can be either a square wave modulation signal or white noise.
Reducing dead band can comprise predetermining and controlling the time at which the bi-directional pulses are launched so that they do not cross at any component within the laser cavity that could cause scattering of the light. Controlling the pulse crossing point can comprise control via traveling wave gate or traveling wave gain. Controlling the launch time can comprise controlling with unidirectional amplification such as with an optical parametric oscillator or with doped gain fibers pumped by a short pulse. Controlling the launch time can alternatively comprise controlling the timing in either direction by a directional optical gate. Controlling with a directional optical gate can comprise controlling a modulator within a fiber laser by providing a first electrical pulse to switch on the modulator and allowing light to pass through the gate in one direction, and then providing a second electrical pulse to switch the modulator, thereby allowing light to then pass in the opposite direction to the first. Alternatively, controlling by unidirectional amplification can comprise the steps of providing a ring laser with bi-directional synchronous pumping and controlling the pulse crossing point with optical delay lines. The ring laser can comprise an optical parametric oscillator that is pumped in two directions by a pulsed laser and preferably the optical parametric oscillator is periodically poled. The method further comprises the steps of using the optical parametric oscillator near degeneracy and reducing the fundamental noise limit by using the idler output instead of the signal through output mirrors.
The present invention is further a method of detecting magnetic susceptibility comprising providing a bi-directional pulsed ring laser, a modulator, an oscillator, and a coil; and detecting the change in phase between the arrival times of the bi-directional pulses at the modulator and the electrical signal of the modulator to determine the change in frequency of the coil. The method further comprises the step of synchronizing the coil frequency and oscillator frequency to the laser cavity repetition frequency. Preferably, the method further comprises achieving the maximum beat frequency in the laser. Detecting comprises sensing a perturbation of the coil and oscillator; changing the phase between the bi-directional pulses at the modulator and the electrical signal of the modulator due to the perturbation; measuring the beat frequency of the laser due to the change in phase; and converting the measured beat frequency into a measure of the coil magnetic susceptibility. Additionally, the laser comprises a fiber laser.
The present invention is also an apparatus for a bi-directional pulsed ring laser comprising a laser, a pump, and a substance having an index of refraction that is light intensity-dependent located in proximity to a beam waist of the laser cavity. The ring laser further comprises an aperture inserted in the laser cavity where beam diameter decreases with intensity to produce bi-directional short light pulses. The laser of the ring laser comprises either a Ti:sapphire laser, an Nd:vanadate laser, or a Cr:LISAF laser. The nonlinear substance can comprise nonlinear crystals or fluids that have an index of refraction that is light intensity-dependent. The fluid can be provided in glass cells or in free-flowing jets.
The ring laser of the present invention further provides means for reducing dead band. Preferably the means for reducing dead band comprises electronic non-constant modulation, which provides a beat frequency spectrum in addition to the one due to rotation alone. The electronic non-constant modulation comprises a signal that is symmetrically switched between opposite polarities, such as square wave modulation. Alternatively, the electronic non-constant modulation can be white noise.
The means for reducing dead band can comprise means for predetermining and controlling the time at which the bi-directional pulses are launched such that they do not cross at any component within the laser cavity that could cause scattering of the light. These means can comprise traveling wave gate or traveling wave gain. Unidirectional amplification can be used such as optical parametric oscillators or doped gain fibers pumped by a short pulse. The traveling wave gate can comprise a directional optical gate for controlling the timing in either direction. This directional optical gate can comprise a modulator within a fiber laser wherein a first electrical pulse switches on the modulator and allows the light to pass through in one direction, and then a second electrical pulse switches the modulator to allow light to then pass in the opposite direction to the first. This continues back and forth as necessary to control the bi-directional pulses.
The unidirectional amplification can alternatively comprise a ring laser with bi-directional synchronous pumping and optical delay lines for controlling the pulse crossing point. This ring laser can comprise an optical parametric oscillator that is pumped in two directions by a pulsed laser, and is preferably periodically poled and is operated near degeneracy.
The present invention is further a magnetic susceptibility detector comprising a bi-directional pulsed ring laser, a modulator, an oscillator, and a coil. Preferably, the frequency of each of the coil and the oscillator are synchronized with the cavity repetition frequency of the laser, and preferably the laser is operating at maximum beat frequency. In the detector, the coil is sensitive to perturbations that cause a change in the phase between the bi-directional pulses of the laser at the modulator and the electrical signal of the modulator, and the beat frequency of the laser is changed due to the change in phase thereby providing a measure of the coil magnetic susceptibility. The laser can be a fiber laser.
A primary object of the present invention is to provide a bi-directional short pulse ring laser with a nonlinear crystal, or other substance, having negligible frequency lock-in.
A primary advantage of the present invention is extremely sensitive measurements of magnetic susceptibility, displacement, voltage, and more.